1. Field of the Invention
The present invention relates to a damper for vibratively supporting parts contained in a vibrating system of a speaker.
2. Description of the Related Art
FIG. 5 is a sectional view showing a general structure of a speaker.
In FIG. 5, a damper 1 is located between a voice coil bobbin 2 and a frame 8. The damper 1 supports the voice coil bobbin 2 within a magnetic gap defined between the damper and a plate 5 that is mounted on a yoke 3 and a magnet 4 in a state that the voice coil bobbin 2 is vibratable in its axial direction.
In FIG. 5, reference numeral 6 designates a vibration plate and 7 designates an edge.
In some speakers each having such a structure, a butterfly damper as shown in FIG. 6 is used for the damper 1.
In the butterfly damper 1, an outer frame 1A to be mounted on the frame 8 and an inner frame 1B to be mounted on the voice coil bobbin 2 are connected by arm parts 1C, which are integrally formed with those frames. The butterfly damper is injection molded of thermoplastic resin.
Conventionally, polybutylene terephthalate (PBT) whose tensile strength is 46 MPa, tensile elongation is 200%, flexural modulus is 2200 MPa, and internal loss is 0.014, is used for the thermoplastic resin forming the butterfly damper 1.
The butterfly damper 1 may be formed to have partial non-uniform sections, and has a satisfactory follow-ability for a vibration of the voice coil bobbin 2.
In the conventional butterfly damper 1 molded of the polybutylene terephthalate (PBT), an unnecessary resonance sometimes occurs in the arm parts 1C at a frequency higher than the lowest resonance frequency fo of the speaker. As the resonance grows, the quality of a sound generated by the speaker is adversely affected by the resonance. In an extreme case, the coupling parts between the arm parts 1C and the outer and inner frames 1A and 1B will be broken.
More exactly, in the butterfly damper 1 made of the material having physical property values as mentioned above, peaks appear in the impedance curve at sound pressure peak levels within a frequency range from 1000 to 2000 Hz, as shown in FIG. 7. At this time, there is a danger that a resonance sound is generated or the arm parts 1C are broken.
The butterfly damper is made of polybutylene terephthalate (PBT) whose tensile strength is 25 MPa, tensile elongation is 300%, flexural modulus is 800 MPa, and internal loss is 0.044. The resonance frequency of the damper shifts to a low frequency region, but this does not lead to the complete prevention of the generation of the unwanted resonance.
There is a damper whose surface is coated with damping compound, for example, in order to prevent the resonance of the butterfly damper 1. The coating of the damping compound results in increase of the number of the steps of manufacturing the butterfly damper, and hence increase of the cost to manufacture.